The present invention relates generally to the art of gaskets used in coupling systems for connecting any of a variety of relatively large plastic parts where large tolerances and exaggerated surface irregularities tend to exist. More particularly, the present invention pertains to the construction of gaskets used to seal relatively large pipe, tubing, fittings, tanks, vessels and the like, which are manufactured as components and assembled and sealed separately in the field. Although the present invention has application to virtually any type plastic component, it is particularly well suited for use in connection with larger bell and spigot type coupling joints between adjoining sections of plastic pipe, where watertight integrity is oftentimes especially critical. For this reason, the following discussion will focus primarily on the construction and use of the present invention in relation to bell and spigot type couplings joints for pipe, it being understood that the principles set forth herein apply equally to seals utilized in the coupling joints of other plastic components.
Plastic piping is growing in acceptance for use in water, sanitary, chemical, and storm sewer transfer applications. Under current standards established by the American Society of Testing and Materials (ASTM), it is required that corrugated high density polyethylene (HDPE) pipe intended for use in certain drain and gravity sewage applications achieve a watertight joint to a pressure of 10.8 psi (74.5 kPa) during laboratory testing. Certain field tests, however, may also be conducted on installed watertight pipe. Such filed tests typically comprise a short term hydrostatic or air pressure test where a specified amount of leakage is allowed. The applied pressure is typically 3.50-4.50 psi (24.1-31.0 kPa), and the length of test and allowable leakage is dependent on the specific project, diameter of pipe and length of run. A common criterion is 200 gallons per inch diameter per mile of pipe per day. These are standard storm sewer requirements; however, when you begin to approach sanitary sewer applications, or low head irrigation lines (under 20 psi) (137.9 kPa)), or other industrial applications, it becomes much more important to have zero or near zero leakage.
Many plastic piping applications are now moving toward minimal leakage requirements or low pressure operating conditions. Specifically, many sanitary sewerage, water transfer or storage, and industrial piping applications require zero leakage to prevent environmental damage, resource wastes, and to mitigate treatment costs of effluent leaking into or out of pipe joints. While many different pipe materials have been used for these applications, including without limitation, solid wall High Density Polyethylene (HDPE), Polyvinyl Chloride (PVC) pipe, etc., corrugated plastic pipe is becoming increasingly popular due to the durability, light weight, ease of handling, and lower cost structure. Particularly in watertight gravity-flow drainage, water storage and sewage applications, HDPE or polypropylene (PPE) corrugated plastic pipe are now in common use.
Traditionally, the pipe joints of these products are typically comprised of a bell and a spigot, with a gasket used to seal the connection between the two joint ends. These types of joints are generally suitable for standard storm drainage applications where moderate leakage is tolerable; however, they are not well suited for applications where low pressure conditions exist or where leakage is a critical concern. In order for a gasketed bell and spigot joint to resist leakage, the gasket must exert a relatively considerable compressive force against both the spigot and bell. If a spigot is not properly inserted into a bell, there is a risk of the gasket becoming dislodged or “rolled,” thereby preventing the necessary amount of compressive force to create a leak resistant joint. Even if the joints are properly assembled, the significant compressive force can cause “creep” in the plastic bell or spigot, whereby the bell may creep outward and the spigot inward, thereby relieving the gasket compression and allowing leakage through the joint.
Electrofusion technology has previously been utilized in jointing systems for plastic pipes to create a welded joint between two abutting pipe ends. While generally suitable for piping systems of smaller diameters, such technology has proven to be more difficult to employ and generally incompatible with larger diameter profile wall plastic pipe, and bell and spigot jointing systems, due primarily to manufacturing tolerances, pipe design and cost. Variations in pipe diameter from nominal, degrees of ovality and alignment problems, all of which are difficult to eliminate, make the use of such technology extremely difficult for joining pipes of larger diameters.
Consequently, it is evident that in order to achieve a positive seal and provide a substantially leak-proof bell and spigot joint that is capable of use in low pressure conditions, or where leakage is a critical concern, a new approach is necessary. There is a distinct need in the plastic pipe industry for a bell and spigot type jointing system that will address the dimensional and gasket-related issues associated with larger diameter pipes, and achieve such a positive seal and substantially leak-proof pipe joint.